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<H1> 9.4 Protected Units and Protected Objects</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3362"></A><A NAME="I3363"></A><A NAME="I3364"></A><A NAME="I3365"></A>A
<I>protected object</I> provides coordinated access to shared data, through
calls on its visible <I>protected operations</I>, which can be <I>protected
subprograms</I> or <I>protected entries</I>. <A NAME="I3366"></A><A NAME="I3367"></A><A NAME="I3368"></A>A
<I>protected unit</I> is declared by a <I>protected declaration</I>,
which has a corresponding <FONT FACE="Arial, Helvetica">protected_body</FONT>.
A protected declaration may be a <FONT FACE="Arial, Helvetica">protected_type_declaration</FONT>,
in which case it declares a named protected type; alternatively, it may
be a <FONT FACE="Arial, Helvetica">single_protected_declaration</FONT>,
in which case it defines an anonymous protected type, as well as declaring
a named protected object of that type. <A NAME="I3369"></A></DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_type_declaration<A NAME="I3370"></A>
::= </FONT><BR>
<B>protected</B> <B>type</B> <A NAME="I3371"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> [<A NAME="I3372"></A><FONT FACE="Arial, Helvetica">known_discriminant_part</FONT>] <B>is</B> <A NAME="I3373"></A><FONT FACE="Arial, Helvetica">protected_definition</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">single_protected_declaration<A NAME="I3374"></A>
::= </FONT><BR>
<B>protected</B> <A NAME="I3375"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>is</B> <A NAME="I3376"></A><FONT FACE="Arial, Helvetica">protected_definition</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_definition<A NAME="I3377"></A>
::= </FONT><BR>
{ <A NAME="I3378"></A><FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT> }<BR>
[ <B>private</B><BR>
{ <A NAME="I3379"></A><FONT FACE="Arial, Helvetica">protected_element_declaration</FONT> } ]<BR>
<B>end</B> [<I>protected_</I><A NAME="I3380"></A><FONT FACE="Arial, Helvetica">identifier</FONT>]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5/1</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_operation_declaration<A NAME="I3381"></A>
::= </FONT><A NAME="I3382"></A><FONT FACE="Arial, Helvetica">subprogram_declaration</FONT><BR>
| <A NAME="I3383"></A><FONT FACE="Arial, Helvetica">entry_declaration</FONT><BR>
| <A NAME="I3384"></A><FONT FACE="Arial, Helvetica">aspect_clause</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_element_declaration<A NAME="I3385"></A>
::= </FONT><A NAME="I3386"></A><FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT><BR>
| <A NAME="I3387"></A><FONT FACE="Arial, Helvetica">component_declaration</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_body<A NAME="I3388"></A>
::= </FONT><BR>
<B>protected</B> <B>body</B> <A NAME="I3389"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>is</B><BR>
{ <A NAME="I3390"></A><FONT FACE="Arial, Helvetica">protected_operation_item</FONT> }<BR>
<B>end</B> [<I>protected_</I><A NAME="I3391"></A><FONT FACE="Arial, Helvetica">identifier</FONT>];</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8/1</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_operation_item<A NAME="I3392"></A>
::= </FONT><A NAME="I3393"></A><FONT FACE="Arial, Helvetica">subprogram_declaration</FONT><BR>
| <A NAME="I3394"></A><FONT FACE="Arial, Helvetica">subprogram_body</FONT><BR>
| <A NAME="I3395"></A><FONT FACE="Arial, Helvetica">entry_body</FONT><BR>
| <A NAME="I3396"></A><FONT FACE="Arial, Helvetica">aspect_clause</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="SyntaxIndented">If a <I>protected_</I><FONT FACE="Arial, Helvetica">identifier</FONT>
appears at the end of a <FONT FACE="Arial, Helvetica">protected_definition</FONT>
or <FONT FACE="Arial, Helvetica">protected_body</FONT>, it shall repeat
the <FONT FACE="Arial, Helvetica">defining_identifier</FONT>. </DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3397"></A>A protected declaration requires
a completion, which shall be a <FONT FACE="Arial, Helvetica">protected_body</FONT>,
and every <FONT FACE="Arial, Helvetica">protected_body</FONT> shall be
the completion of some protected declaration. </DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">protected_definition</FONT>
defines a protected type and its first subtype. <A NAME="I3398"></A>The
list of <FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT>s
of a <FONT FACE="Arial, Helvetica">protected_definition</FONT>, together
with the <FONT FACE="Arial, Helvetica">known_discriminant_part</FONT>,
if any, is called the visible part of the protected unit. <A NAME="I3399"></A>The
optional list of <FONT FACE="Arial, Helvetica">protected_element_declaration</FONT>s
after the reserved word <B>private</B> is called the private part of
the protected unit. </DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3400"></A>The elaboration of a protected
declaration elaborates the <FONT FACE="Arial, Helvetica">protected_definition</FONT>.
<A NAME="I3401"></A>The elaboration of a <FONT FACE="Arial, Helvetica">single_protected_declaration</FONT>
also creates an object of an (anonymous) protected type. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3402"></A>The elaboration of a <FONT FACE="Arial, Helvetica">protected_definition</FONT>
creates the protected type and its first subtype; it also includes the
elaboration of the <FONT FACE="Arial, Helvetica">component_declaration</FONT>s
and <FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT>s
in the given order.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3403"></A>As part of the initialization
of a protected object, any per-object constraints (see <A HREF="RM-3-8.html">3.8</A>)
are elaborated. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3404"></A>The elaboration of a <FONT FACE="Arial, Helvetica">protected_body</FONT>
has no other effect than to establish that protected operations of the
type can from then on be called without failing the Elaboration_Check.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The content of
an object of a given protected type includes: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>The values of the components of the protected object, including
(implicitly) an entry queue for each entry declared for the protected
object; </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I3405"></A>A representation of the state of the
execution resource <I>associated</I> with the protected object (one such
resource is associated with each protected object). </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Normal"> The execution resource associated with a protected
object has to be acquired to read or update any components of the protected
object; it can be acquired (as part of a protected action -- see <A HREF="RM-9-5-1.html">9.5.1</A>)
either for concurrent read-only access, or for exclusive read-write access.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3406"></A><A NAME="I3407"></A>As the
first step of the <I>finalization</I> of a protected object, each call
remaining on any entry queue of the object is removed from its queue
and Program_Error is raised at the place of the corresponding <FONT FACE="Arial, Helvetica">entry_call_statement</FONT>.
</DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>13 Within the declaration
or body of a protected unit, the name of the protected unit denotes the
current instance of the unit (see <A HREF="RM-8-6.html">8.6</A>), rather
than the first subtype of the corresponding protected type (and thus
the name cannot be used as a <FONT FACE="Arial, Helvetica">subtype_mark</FONT>).
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>14 A <FONT FACE="Arial, Helvetica">selected_component</FONT>
can be used to denote a discriminant of a protected object (see <A HREF="RM-4-1-3.html">4.1.3</A>).
Within a protected unit, the name of a discriminant of the protected
type denotes the corresponding discriminant of the current instance of
the unit.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>15 A protected type is a
limited type (see <A HREF="RM-7-5.html">7.5</A>), and hence has neither
an assignment operation nor predefined equality operators.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>16 The bodies of the protected
operations given in the <FONT FACE="Arial, Helvetica">protected_body</FONT>
define the actions that take place upon calls to the protected operations.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>25</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>17 The declarations in the
private part are only visible within the private part and the body of
the protected unit. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>26</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of declaration
of protected type and corresponding body:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>27</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> <B>type</B> Resource <B>is</B><BR>
<B>entry</B> Seize;<BR>
<B>procedure</B> Release;<BR>
<B>private</B><BR>
Busy : Boolean := False;<BR>
<B>end</B> Resource;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>28</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> <B>body</B> Resource <B>is</B><BR>
<B>entry</B> Seize <B>when not</B> Busy <B>is</B><BR>
<B>begin</B><BR>
Busy := True;<BR>
<B>end</B> Seize;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>29</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Release <B>is</B><BR>
<B>begin</B><BR>
Busy := False;<BR>
<B>end</B> Release;<BR>
<B>end</B> Resource;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>30</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of a single
protected declaration and corresponding body:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>31</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> Shared_Array <B>is</B><BR>
<I>-- Index, Item, and Item_Array are global types</I><BR>
<B>function</B> Component (N : <B>in</B> Index) return Item;<BR>
<B>procedure</B> Set_Component(N : <B>in</B> Index; E : <B>in</B> Item);<BR>
<B>private</B><BR>
Table : Item_Array(Index) := (others => Null_Item);<BR>
<B>end</B> Shared_Array;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>32</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> <B>body</B> Shared_Array <B>is</B><BR>
<B>function</B> Component(N : <B>in</B> Index) <B>return</B> Item <B>is</B><BR>
<B>begin</B><BR>
<B>return</B> Table(N);<BR>
<B>end</B> Component;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>33</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Set_Component(N : <B>in</B> Index; E : <B>in</B> Item) <B>is</B><BR>
<B>begin</B><BR>
Table(N) := E;<BR>
<B>end</B> Set_Component;<BR>
<B>end</B> Shared_Array;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>34</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Examples of protected
objects:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>35</FONT></DIV>
<DIV Class="Examples"><TT>Control : Resource;<BR>
Flags : <B>array</B>(1 .. 100) <B>of</B> Resource;</TT></DIV>
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